High data rate (e.g., 100M, 1 G, and 10 G) Ethernet PHY systems as specified in IEEE 802.3 are designed to perform over categorized cables with predefined specifications. These categorized cables show a moderate amount of Inter Symbol Interference (ISI). Some form of ISI mitigation, and in particular, various forms of Decision Feedback Equalizers (DFE), are often used to combat ISI. DFE may comprise feedforward and feedback filters. In order to train the DFE and other signal processing blocks, the transmitter sends an idle signal before data communication starts. The idle signal is generated using a rather long scrambler specified in IEEE 802.3. The DFE is trained on the idle signal using decisions made at the equalizer output. This works well on the categorized cables specified in IEEE 802.3. When it comes to channels with severe ISI however, the Ethernet PHY specified in IEEE 802.3 does not, and is not intended, to work.
Multi carrier modulations and in particular various forms of coded Orthogonal Frequency Division Multiplexing (OFDM) are often used to operate over channels with more severe ISI. Despite many advantages offered by OFDM, systems using OFDM may suffer from higher power consumption and/or complexity in some applications. In addition, the system throughput may be reduced due to signaling overhead.